The crosslinking of a rubber having in the molecule thereof a carbon-carbon double bond such as EPDM, NR, and NBR is generally performed using sulfur. A practical vulcanizing temperature for crosslinking these rubbers using sulfur is necessary to be 150° C. to 200° C. Therefore, these crosslinked rubbers have such a concern that rubber properties thereof are lowered by a thermal degradation during crosslinking as the vulcanizing temperature becomes higher. Moreover, for heating a rubber or the like, much energy is required. Therefore, it is studied to lower the vulcanizing temperature, and as one method therefor, a method of using a combination of various accelerators or the like is studied. However, by this method, the vulcanizing time becomes undesirably longer, so that such a study does not become commercially practical.
On the other hand, as a crosslinker for crosslinking a rubber at a low temperature, Patent Document 1 describes a polynitrile oxide having, in the molecule thereof, a plurality of nitrile oxide groups such as mesitylene dinitrile oxide (MDNO).